Behavioral sensitization, the progressive and enduring enhancement of certain behaviors following repetitive drug use, augments rodent locomotion in a long-standing fashion. The same dopamine pathways playing an important role in drug dependence and psychosis also play a critical role in sensitization. The role of individual dopamine receptor subtypes in sensitization, however, has not been clearly identified. D3 dopamine receptor stimulation inhibits rodent locomotion. D3 receptor activity may be regulated through expression of an alternatively spliced, truncated receptor isoform (termed "D3nf") altering receptor localization and function via dimerization with the full-length subunit. The central hypotheses for our research are that 1.) repetitive D3 receptor stimulation contributes to development of sensitization through decreased responsivity of D3 receptor-mediated locomotor inhibition; and 2.) increased D3nf expression directs altered receptor localization and subsequent release of D3-receptor mediated inhibition, contributing to expression of sensitization. We will test these hypotheses with the following Specific Aims. In Specific Aim 1,we identify the role of D3 receptors in behavioral sensitization to amphetamine. We test the hypothesis that a homeostatic, compensatory response to D3 receptor stimulation contributes to altered D3 receptor splicing and the development of sensitization by determining the effect of dopamine receptor agonists and antagonists on development of sensitization and D3 receptor isoform expression. In Specific Aim 2, we will evaluate behavioral response to D3 receptor antagonists following chronic drug administration. We will measure the behavioral response to D3 receptor blockade, using D3-selective drugs as a tool to measure D3 receptor function. This aim tests the hypothesis that expression of sensitization results in part from release of D3-mediated inhibition, therefore resulting in decreased response to D3 receptor antagonist. In Specific Aim 3, we will determine the consequences of chronic amphetamine administration on D3 dopamine receptor expression. We will measure expression of both full-length D3 receptor and D3nf mRNA, and also measure D3 receptor internalization. This aim tests the hypothesis that chronic amphetamine administration increases D3nf expression, thereby inhibiting full-length D3 receptor function by internalizing the full-length receptor. We expect to demonstrate increased D3nf expression, and increased D3 receptor internalization, following sensitization. This finding would suggest alternative splicing pathways as a novel intervention to prevent sensitization, as well as restore D3-mediated inhibitory function, and would also suggest a biological function for D3nf. Collectively; these studies provide a multi-faceted test of a novel hypothesis of the mechanism underlying long-standing changes in limbic-mediated behaviors. These outcomes may suggest new interventions for neuropsychiatric conditions in which dopamine is known to play an important role, including psychosis and drug dependence. Significantly, these studies may also elucidate a previously unrecognized mechanism regulating receptor desensitization and trafficking relevant to other receptor systems and pathological conditions.